A simple adapter to add a filter with easily interchangeable filter material to a snorkel mask easily obtainable online.

I modeled this adapter to fit this snorkel mask from Amazon:

https://www.amazon.com/TINMIU-Efficient-Snorkeling-Detachable-Anti-Fogging/dp/B083DM1NGH/

There are many other similar snorkel masks on Amazon, some of which are probably the same model, but available at a lower price.  

The adapter works quite well.  The threads may need some adjustment.  I put thread sealing tape on my model to make sure I had a good connection.  I printed an earlier revision, but the threads were too big, and so I erred on the side of caution, so the threads are looser now, but I'm happy with the way everything fits together off of my printer.

One limitation of this mask is that it cannot filter the outgoing air, since the outlet valves do not transport the air back up to the top.  This is only because this is the snorkel mask I had on hand though, and it should be fairly easy to adapt this design to fit a mask with inlet and outlet valves at the top if you have a mask to measure and test with.